The present invention relates to radiofrequency or microwave ablation and in particular to a method of monitoring tissue ablation concurrent with the ablation process.
Elastography is an imaging modality that reveals the stiffness properties of tissues, for example, axial strain, lateral strain, Poisson's Ratio, Young's Modulus, or other common stiffness measurements. The stiffness measurements may be output as quantitative values or mapped to a gray or color scale to form a picture over a plane or within a volume.
Generally, stiffness is deduced by monitoring tissue movement under an applied force or deformation. The monitoring may be done by any medical imaging modality including computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonic imaging. Elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount that the tissue yields under pressure.
In “dynamic” elastography, a low frequency vibration is applied to the tissue and the velocity of the resulting compression waves is measured, for example, using ultrasonic Doppler detection. In “quasi-static” elastography, two images of the tissue are obtained at different states of compression, typically using the ultrasonic transducer as a compression paddle. Displacement of the tissue between the two images is used to deduce the stiffness of the tissue.
U.S. Pat. No. 7,166,072, assigned to the same assignee as the present invention and hereby incorporated by reference, describes a novel technique for monitoring a radiofrequency ablation using quasi-static elastography. Radiofrequency or microwave ablation is a process for treating tumors or the like which employs one or more of electrodes inserted percutaneously to the site of a tumor. Ionic heating of the tissue induced by radiofrequency fields in the tissue kills tumor cells and produces a hardened lesion. This lesion, being much stiffer than the surrounding tissue, may be monitored by quasi-static elastography using the ablation electrode as the compression device. Adhesion between the ablated tissue and the electrode allows the source of the compression to be at the site of the tumor (as opposed to external compression to the patient) providing a more accurate characterization of the stress field near the tumor and, accordingly, substantially improved elastographic measurement. As used herein, the term “high-frequency ablation” will be used for ablation using either radiofrequency or microwave frequency electrical energy.